1. Field of the Invention
The invention relates to a test method for estimating the level of residual surfactant on the skin.
2. The Related Art
Personal cleansing products are frequently marketed based on their degree of mildness to the skin of the user. Unfortunately, many surfactants in common use tend bind to the skin proteins, which can result in skin irritation. Moreover different surfactants bind to skin proteins to varying degrees. Cleansers that do not leave residual surfactants on the skin are therefore more desirable to many consumers. At least one test has been disclosed for evaluating the amount of dye and/or pigment binding to skin and evaluating the extent of skin adsorption of surfactant which interferes with dye and/or pigment binding. However there has not been a convenient way for the consumer to ascertain how a particular cleanser will interact with their skin using a particular cleansing method until now.
A diagnostic test is disclosed by Imokawa and Mishima, Contact Dermatitis 5:357-366, (1979) which uses dye to determine whether or not skin has appreciable amounts of adsorbed surfactant. The disclosed test uses dye solutions which are not convenient or appropriate when developing a convenient method intended for consumer use.
A convenient method for detecting and quantifying residual surfactants has been discovered that employs soluble or dispersible solid or semisolid films as dye and/or pigment carriers where the dye and/or pigment(s) have an affinity for either the skin or the surfactant left on the skin. Although not wishing to be bound to the following theory, it is believed that in a first case where a dye and/or pigment has an affinity for the skin but not for the residual surfactant of interest, application of the dye and/or pigment to skin with residual surfactant will result in skin with less coloration then to skin without the residual surfactant. In a second case where a dye and/or pigment has an affinity for the residual surfactant of interest but not for the skin, application of the dye and/or pigment to skin with residual surfactant will result in skin with more coloration then to skin without the residual surfactant. Both cases assume that neither the surfactant nor the reagent which removes the film/coating alters the skin itself in a way that changes the skin's propensity to interact with a given dye or pigment independently of the amount of surfactant adsorbed onto the skin.